1. Field of the Invention
The present invention relates to a ferrule for an optical fiber, capable of transmitting linearly polarized light, and an optical fiber connector for such optical fibers using such a ferrule.
2. Description of the Background Art
A ferrule is most commonly utilized in an optical fiber connector for connecting an optical fiber with other optical fibers or optical elements, but other uses such as those in a collimator, in an LD module, and in an optical fiber terminal are also known.
Conventionally, there are two types of ferrule both of which are suitable for single mode optical fibers. One type is a zirconia ferrule shown in FIG. 1 comprising an insertion portion 1 made of zirconia ceramic, with a puncture 2 for an optical fiber, a flange portion 3 made of stainless steel, with an open end 4 for receiving a coated optical fiber, having a tapering inner diameter between a part including the open end 4 which has a smaller diameter and another part including another end to be attached to the insertion portion 1 which has a larger diameter, and a keyway 5 for receiving connection keys of a plug housing in which the ferrule is to be installed.
Another type is a capillary type ferrule shown in FIG. 2 comprising an insertion portion 11 with an open end 12 for receiving a coated optical fiber, a capillary 13 made of alumina ceramic, with a puncture 14 for an optical fiber, attached to another end of the insertion portion, and a keyway 15 for receiving connection keys of a plug housing in which the ferrule is to be installed. This type of ferrule may also have a hole 16 at a foot of the keyway 15 whose role will be explained later.
Among these two types, the zirconia type of FIG. 1 is usually a preferred choice because of its high reliability due to higher hardness and bending strength of the insertion portion 1, its inexpensiveness due to its simple configuration, and a stability of its operation. This last point is due to the fact that zirconia ceramic has a lower Young's modulus than alumina ceramic, so that by polishing the ends of ferrules into round shape and pressing each other along their central axes the fiber cores can be maintain in a physical contact by the deformation of the edges of ferrules even when the fibers happened to be drawn into the ferrules.
Meanwhile, there are single mode polarization maintaining optical fibers (referred hereafter as SMPM fibers) which can transmit linearly polarized light with its polarization state maintained. Namely, the SMPM fiber can maintain the polarization state of the polarized light which is linearly polarized along the birefringent axes of the optical fiber. This property is usually evaluated in terms of a parameter called an extinction ratio which is a ratio of an output power of the linearly polarized light entered in a direction of a main axis of the birefringent axes with respect to that in a direction perpendicular to the main axis. This extinction ratio may take a value in a range of -40 to -50 dB for a short SMPM fiber of about 10 m long. In such a SMPM fiber, when the internal stresses are produced by bending of the optical fiber, the extinction ratio can increases due to the appearance of other birefringence inside the optical fiber.
Now, in attaching the optical fiber to the zirconia ferrule of FIG. 1, the epoxy adhesives A are dropped from the open end 4 of the flange portion 3 as shown in FIG. 3(A), and the air inside the flange portion 3 are extracted through the puncture 2 of the insertion portion 1 by vacuum pumping as shown in FIG. 3(B), to fill the inside of the ferrule with adhesives A. The optical fiber is then inserted into the ferrule, and fixed by heating up to stiffen the adhesives A.
In this process of filling the adhesives, the air bubbles B frequently appears near the connection between the insertion portion 1 and the flange portion 3 due to the tapering inner diameter of the flange portion 3 in this region, as shown in FIG. 3(C). When the stiffening of the adhesives A is carried out with such bubbles B near the connection between the insertion portion 1 and the flange portion 3, the volume contraction accompanying the stiffening of the inhomogeneously distributed adhesives A causes an extraneous stress exerted on the optical fiber 6, as shown in FIG. 3(D). In FIG. 3(D), the bending of the optical fiber 6 is exaggerated for clarify, and this optical fiber 6 stems from the coated optical fiber 7 which is inserted from the open end 4 of the flange portion 3. As mentioned above, such an extraneous stress is not problematic for single mode optical fibers, but it can cause an increase of the extinction ratio for the SMPM fibers.
In case of the capillary type ferrule of FIG. 2, there may be the hole 16 provided at the foot of the keyway 15 for the purpose of removing the air inside the insertion portion 11 as mentioned above, but this hole 16 cannot be made in the vicinity of the connection of the insertion portion 11 and the capillary 13 as such region is a part of a portion to be inserted into an alignment sleeve of an adaptor in which two ferrules are contacted each other, so that the air bubbles appearing in this region cannot be removed.
Thus, conventionally, the SMPM fiber suffers from the deterioration of the extinction ratio occurring when attached to the ferrule in the optical fiber connector.
As for the optical fiber connector, a most common type is an FC type optical fiber connector shown in FIG. 4. In this FC type optical fiber connector, an optical fiber stemming from a coated optical fiber 20 is attached at a center of an end face of a ferrule 21 installed into a plug housing 22 and inserted into an alignment sleeve 23 in which the other end face of the ferrule 21 is contacted with an end face of another ferrule inserted from the opposite side of the alignment sleeve 23, as the ferrule 21 is pressed by a spring 24 along its central axis.
In such an optical fiber connector for single mode optical fiber, the connection keys of the plug housing are engaged with the keyway of the ferrule such that the rotation of the ferrule is restricted so as to be able to contact the optical fibers at a desired relative angle around their central axes, as shown in FIG. 5(A).
Furthermore, in such an optical fiber connector for single mode optical fiber, the ferrule is not rigidly fixed with respect to the plug housing, in order to tolerate some external stress and improve the stability. This is achieved by providing about 0.2 mm clearance between the connection keys of the plug housing and the keyway of the ferrule as shown in FIG. 5(B).
However, when such an optical fiber connector is used for SMPM fibers, the fluctuation of the relative angle of the optical fibers due to this clearance can cause a deterioration of the extinction ratio as a consequence of the deviated optical fiber angle relation. In general, this relative angle is required to be within a range of 0.5.degree. for the extinction ratio of -40 dB, or a range of 2.degree. for the extinction ratio of -30 dB.
In the FC type optical fiber connector with about 0.2 mm clearance between the connection keys of the plug housing and the keyway of the ferrule, for the ferrule with the flange portion of outer diameter 4.6 mm, the maximum deviation of the relative angle is: EQU tan.sup.-1 (0.2/2.3).apprxeq.5.degree.
for which the extinction ratio is: EQU 10 log(tan.sup.2 (5.degree.)).apprxeq.-21 dB
which is considered not satisfactory in practice.
Moreover, in the FC type optical fiber connector of FIG. 4, the torque for rotating the ferrule is produced by tightening of the coupling nut around the plug housing 22 at a time of coupling, so that the extinction ratio can be further deteriorated.
FIG. 6 shows the extinction ratios as a function of a number of coupling and uncoupling, obtained by trials using an FC type optical fiber connector for SMPM fibers. As can easily be seen from this FIG. 6, the angle of the ferrule is randomly changed at each coupling so that the extinction ratio fluctuates very largely, which implies that the operation is not stable.